Positive tone bi-layer method

ABSTRACT

Methods of patterning a substrate including creating a multi-layered structure by forming, on the substrate, a patterned layer having protrusions and recessions are described. A polymerizable material composition is dispense on the patterned layer defining a conformal layer, with the multi-layered structure having a crown surface facing away from the substrate. Portions of the multi-layered structure are removed to expose regions of the substrate in superimposition with the protrusions, while forming a hard mask in areas of the crown surface in superimposition with the recessions.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation of U.S. application Ser. No.11/844,824 filed Aug. 24, 2007, now U.S. Pat. No. 7,670,953. U.S.application Ser. No. 11/844,824 is a continuation of U.S. applicationSer. No. 11/611,287 filed Dec. 15, 2006 (now U.S. Pat. No. 7,323,417),which is a continuation of U.S. application Ser. No. 10/946,570 filedSep. 21, 2004 (now U.S. Pat. No. 7,186,656). U.S. application Ser. No.10/946,570 is a continuation-in-part of U.S. application Ser. No.10/850,876 filed on May 21, 2004 (abandoned). U.S. application Ser. No.11/844,824 is also a continuation of application Ser. No. 11/560,928filed Nov. 17, 2006 (now U.S. Pat. No. 7,261,831), which is acontinuation of U.S. application Ser. No. 10/396,615 filed Mar. 25, 2003(now U.S. Pat. No. 7,179,396). All of the aforementioned applicationsand patents are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The field of invention relates generally to micro-fabrication ofstructures. More particularly, the present invention is directed topatterning substrates in furtherance of the formation of structures.

Micro-fabrication involves the fabrication of very small structures,e.g., having features on the order of micro-meters or smaller. One areain which micro-fabrication has had a sizeable impact is in theprocessing of integrated circuits. As the semiconductor processingindustry continues to strive for larger production yields whileincreasing the circuits per unit area formed on a substrate,micro-fabrication becomes increasingly important. Micro-fabricationprovides greater process control while allowing increased reduction ofthe minimum feature dimension of the structures formed. Other areas ofdevelopment in which micro-fabrication has been employed includebiotechnology, optical technology, mechanical systems and the like.

An exemplary micro-fabrication technique is shown in U.S. Pat. No.6,334,960 to Willson et al. Willson et al. disclose a method of forminga relief image in a structure. The method includes providing a substratehaving a transfer layer. The transfer layer is covered with apolymerizable fluid composition. An imprint device makes mechanicalcontact with the polymerizable fluid. The imprint device includes arelief structure formed from lands and grooves. The polymerizable fluidcomposition fills the relief structure, with the thickness of thepolymerizable fluid in superimposition with the lands defining aresidual thickness. The polymerizable fluid composition is thensubjected to conditions to solidify and polymerize the same, forming asolidified polymeric material on the transfer layer that contains arelief structure complimentary to that of the imprint device. Theimprint device is then separated from the solid polymeric material suchthat a replica of the relief structure in the imprint device is formedin the solidified polymeric material. The transfer layer and thesolidified polymeric material are subjected to an environment toselectively etch the transfer layer relative to the solidified polymericmaterial such that a relief image is formed in the transfer layer.Thereafter, conventional etching processes may be employed to transferthe pattern of the relief structure into the substrate.

It is desired to minimize dimensional variations between the patternrecorded in the polymeric material from the pattern transferred into thesubstrate, referred to as transfer distortions. To that end, manyattempts have been made to advance the micro-fabrication technique ofWillson et al. For example, it has been desired to minimize the residualthickness of the solidified polymeric material. The thinner the residualthickness, the greater reduction in transfer distortions. The residualthickness of the solidified polymeric material is proportional to theresidual thickness of the polymerizable fluid. However, the rate atwhich the polymerizable fluid fills the relief structure is inverselyproportional to the cube of the residual thickness of polymerizablefluid. It is manifest that minimizing the transfer distortions increasesthe time required to record the pattern in the substrate. Thus, atradeoff exists between throughput and minimization of transferdistortions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a lithographic system in accordance withthe present invention;

FIG. 2 is a simplified elevation view of a lithographic system, shown inFIG. 1, employed to create a patterned imprinting layer in accordancewith the present invention;

FIG. 3 is a simplified representation of material from which a patternedimprinting layer, shown in FIG. 2, is comprised before being polymerizedand cross-linked in accordance with the present invention;

FIG. 4 is a simplified representation of cross-linked polymer materialinto which the material shown in FIG. 3 is transformed after beingsubjected to radiation in accordance with the present invention;

FIG. 5 is a simplified elevation view of an imprint device spaced-apartfrom the patterned imprinting layer, shown in FIG. 1, after patterningin accordance with the present invention;

FIG. 6 is a simplified elevation view of a lithographic system, shown inFIG. 1, after formation of a multi-layered structure by deposition of aconformal layer, adjacent to the patterned imprinting layer, employing amold in accordance with one embodiment of the present invention;

FIG. 7 is a simplified elevation view after a blanket etch of themulti-layered structure, shown in FIG. 6, after formation of a crownsurface in the conformal layer with portions of the patterned imprintinglayer being exposed in accordance with one embodiment of the presentinvention;

FIG. 8 is a simplified elevation view of the multi-layered structure,shown in FIG. 7, after subjecting the crown surface to an anisotropicetch to expose regions of a substrate in accordance with the presentinvention;

FIG. 9 is a simplified elevation view of material in an imprint deviceand substrate employed with the present invention in accordance with analternate embodiment of the present invention;

FIG. 10 is a simplified elevation view of a lithographic system, shownin FIG. 1, after formation of a multi-layered structure by deposition ofa conformal layer, adjacent to the patterned imprinting layer, employinga planarized mold in accordance with an alternate embodiment of thepresent invention; and

FIG. 11 is a simplified elevation view of a multi-layered structureafter deposition of a conformal layer in accordance with an alternateembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a lithographic system 10 in accordance with oneembodiment of the present invention that includes a pair of spaced-apartbridge supports 12 having a bridge 14 and a stage support 16 extendingtherebetween. Bridge 14 and stage support 16 are spaced-apart. Coupledto bridge 14 is an imprint head 18, which extends from bridge 14 towardstage support 16. Disposed upon stage support 16 to face imprint head 18is a motion stage 20. Motion stage 20 is configured to move with respectto stage support 16 along X and Y axes. A radiation source 22 is coupledto system 10 to impinge actinic radiation upon motion stage 20. Asshown, radiation source 22 is coupled to bridge 14 and includes a powergenerator 23 connected to radiation source 22.

Referring to both FIGS. 1 and 2, connected to imprint head 18 is asubstrate 26 having a patterned mold 28 thereon. Patterned mold 28includes a plurality of features defined by a plurality of spaced-apartrecesses 28 a and projections 28 b. Projections 28 b have a width W₁,and recesses 28 a have a width W₂, both of which are measured in adirection that extends transversely to Z axis. The plurality of featuresdefines an original pattern that is to be transferred into a wafer 31positioned on motion stage 20. To that end, imprint head 18 is adaptedto move along the Z axis and vary a distance “d” between patterned mold28 and wafer 31. Alternatively, or in conjunction with imprint head 18,motion stage 20 may move substrate 26 along the Z-axis. In this manner,the features on patterned mold 28 may be imprinted into a flowableregion of wafer 31, discussed more fully below. Radiation source 22 islocated so that patterned mold 28 is positioned between radiation source22 and wafer 31. As a result, patterned mold 28 is fabricated frommaterial that allows it to be substantially transparent to the radiationproduced by radiation source 22.

Referring to both FIGS. 2 and 3, a flowable region, such as a patternedimprinting layer 34, is disposed on a portion of surface 32 thatpresents a substantially planar profile. Flowable region may be formedusing any known technique such as a hot embossing process disclosed inU.S. Pat. No. 5,772,905, which is incorporated by reference in itsentirety herein, or a laser assisted direct imprinting (LADI) process ofthe type described by Chou et al. in Ultrafast and Direct Imprint ofNanostructures in Silicon, Nature, Col. 417, pp. 835-837, June 2002. Inthe present embodiment, however, flowable region consists of patternedimprinting layer 34 being deposited as a plurality of spaced-apartdiscrete beads 36 of material 36 a on wafer 31, discussed more fullybelow. Patterned imprinting layer 34 is formed from a substantiallysilicon-free material 36 a that may be selectively polymerized andcross-linked to record the original pattern therein, defining a recordedpattern. Material 36 a is shown in FIG. 4 as being cross-linked atpoints 36 b, forming cross-linked polymer material 36 c.

Referring to FIGS. 2, 3 and 5, the pattern recorded in patternedimprinting layer 34 is produced, in part, by mechanical contact withpatterned mold 28. To that end, imprint head 18 reduces the distance “d”to allow patterned imprinting layer 34 to come into mechanical contactwith patterned mold 28, spreading beads 36 so as to form patternedimprinting layer 34 with a contiguous formation of material 36 a oversurface 32. In one embodiment, distance “d” is reduced to allowsub-portions 34 a of patterned imprinting layer 34 to ingress into andfill recesses 28 a.

To facilitate filling of recesses 28 a, material 36 a is provided withthe requisite properties to completely fill recesses 28 a while coveringsurface 32 with a contiguous formation of material 36 a. In the presentembodiment, sub-portions 34 b of patterned imprinting layer 34 insuperimposition with projections 28 b remain after the desired, usuallyminimum distance “d”, has been reached, leaving sub-portions 34 a with athickness, t₁ and sub-portions 34 b with a thickness, t₂. Thickness t₂is referred to as a residual thickness. Thicknesses “t₁” and “t₂” may beany thickness desired, dependent upon the application.

Referring to FIGS. 2, 3 and 4, after a desired distance “d” has beenreached, radiation source 22 produces actinic radiation that polymerizesand cross-links material 36 a, forming cross-linked polymer material 36c. As a result, the composition of patterned imprinting layer 34transforms from material 36 a to material 36 c, which is a solid.Specifically, material 36 c is solidified to provide side 34 c ofpatterned imprinting layer 34 with a shape conforming to a shape of asurface 28 c of patterned mold 28, shown more clearly in FIG. 5, withpatterned imprinting layer 34 having recessions 30 a and protrusions 30b. After patterned imprinting layer 34 is transformed to consist ofmaterial 36 c, shown in FIG. 4, imprint head 18, shown in FIG. 2, ismoved to increase distance “d” so that patterned mold 28 and patternedimprinting layer 34 are spaced-apart.

In a further embodiment, recessions 30 a and protrusions 30 b ofimprinting layer 34 may be formed by such techniques including, but notlimited to, photolithography (various wavelengths including G line, Iline, 248 nm, 193 nm, 157 nm, and 13.2-13.4 nm), e-beam lithography,x-ray lithography, ion-beam lithography, and atomic beam lithography.

Referring to FIG. 6, additional processing is employed to form amulti-layered structure 38 by forming a conformal layer 40 adjacent topatterned imprinting layer 34. One manner in which to form conformallayer 40 is to employ imprint lithography processes, such as thosediscussed above with respect to depositing patterned imprinting layer34. To that end, conformal layer 40 may be formed from a polymerizablematerial similar to that described above with respect to FIGS. 3 and 4,excepting that the material from which conformal layer 40 is formedincludes silicon, i.e., is a silicon-containing polymerizable material.Conformal layer 40 includes first and second opposed sides. First side40 b faces patterned imprinting layer 34 and has a profile complementaryto the profile of the patterned imprinting layer 34. The second sidefaces away from patterned imprinting layer 34 forming normalizationsurface 40 a. Normalization surface 40 a is provided with asubstantially normalized profile, by ensuring that the distances, k₂,k₄, k₆, k₈ and k₁₀, between the apex 30 c, shown in FIG. 5, of each ofthe protrusions 30 b and normalization surface 40 a are substantiallythe same and that the distance, k₁, k₃, k₅, k₇, k₉ and k₁₁ between anadir surface 30 d of each of the recessions 30 a and normalizationsurface 40 a are substantially the same.

One manner in which to provide normalization surface 40 a with anormalized profile, a planarizing mold 128 having a planar surface 128 ais employed to come into contact with conformal layer 40. As mentionedabove, this may be accomplished by moving imprint head 18, shown in FIG.2, along the Z-axis, moving motion stage 20 along the Z-axis, or both.Thereafter, mold 128 is separated from conformal layer 40 and actinicradiation impinges upon conformal layer 40 to polymerize and, therefore,solidify the same. Alternatively, conformal layer 40 may be appliedemploying spin-on techniques. Spin-on deposition of conformal layer 40may be beneficial when recording patterns having numerous features perunit area, i.e., a dense featured pattern.

Referring to FIGS. 6 and 7, a blanket etch is employed to removeportions of conformal layer 40 to provide multi-layered structure 38with a crown surface 38 a. Crown surface 38 a is defined by an exposedsurface 30 e of each of protrusions 30 b and upper surfaces of portions40 c that remain on conformal layer 40 after the blanket etch.

Referring to FIGS. 7 and 8, crown surface 38 a is subjected to ananisotropic etch. The etch chemistry of the anisotropic etch is selectedto maximize etching of protrusions 30 b and the segments of patternedimprinting layer 34, shown in FIG. 6, in superimposition therewith,while minimizing etching of the portions 40 c in superimposition withrecessions 30 a. In the present example, advantage was taken of thedistinction of the silicon content between the patterned imprintinglayer 34 and the conformal layer 40. Specifically, employing a plasmaetch with an oxygen-based chemistry, it was determined that an in-situhardened mask 42 would be created in the regions of portions 40 cproximate to surface 38 a. This results from the interaction of thesilicon-containing polymerizable material with the oxygen plasma. As aresult of the hardened mask 42 and the anisotropicity of the etchprocess, regions 44 of wafer 31 in superimposition with protrusions 30 bare exposed. The width U′ of regions 44 is optimally equal to width W₂,shown in FIG. 2.

Referring to FIGS. 2, 7 and 8, the advantages of this process aremanifold. For example, the relative etch rate between portions 40 c andexposed surfaces 30 e may be in a range of about 1.5:1 to about 100:1due to the presence of the hardened mask 42. As a result, thedimensional width U′ of regions 44 may be precisely controlled, therebyreducing transfer distortions of the pattern into wafer 31.

Referring to FIGS. 1, 5 and 11 additionally, the control of dimensionalwidth U′ becomes relatively independent of residual thickness t₂. Therate at which the polymerizable fluid fills the pattern on mold 28 isinversely proportional to the cube of residual thickness t₂. As aresult, residual thickness t₂ may be selected to maximize throughputwithout substantially increasing transfer distortions. Decoupling of thetransfer distortions from residual thickness t₂ facilitates patterningnon-planar surfaces without exacerbating transfer distortions. This isparticularly useful when mold 28 is deformed due to external forces,such as typically occurs when varying the dimensions of mold 28 wheneffectuating magnification correction. As a result, deformation in moldpatterned imprinting layer 34 may have a profile in which apex 130 c ofprotrusions 130 b are not coplanar and/or nadir surface 130 d ofrecessions 130 a are not coplanar.

To attenuate the transfer distortions that may result from this profile,conformal layer 140 is deposited so that distances, k_(i), between theapex 130 c of each of the protrusions 130 b and normalization surface140 a satisfies the following parameter:|k_(i) _(min) −k_(i) _(max) |

t₃where k_(i) _(min) is smallest value for k_(i) and k_(i) _(max) is thegreatest value for k_(i) and t₃ is the height of protrusion 130 bmeasured between apex 130 c and nadir surface 130 d. Thus, theconstraint on the normalization provided by normalization surface 140 amay be relaxed so as not to require each value of k_(i) to besubstantially identical. To that end, conformal layer 140 may be appliedby either spin-coating techniques or imprint lithography techniques.Thereafter, stage 20 is employed to move substrate 131 along the Z-axisto compress conformal layer 140 against a planar surface, such as mold28. Alternatively, mold 28 may be moved against normalization surface140 a or both.

Finally, forming patterned imprinting layer 34 from a substantiallysilicon-free polymerizable fluid eases the cleaning process of mold 28,especially considering that mold 28 is often formed from fused silica.

Referring to both FIGS. 1 and 2, an exemplary radiation source 22 mayproduce ultraviolet radiation. Other radiation sources may be employed,such as thermal, electromagnetic and the like. The selection ofradiation employed to initiate the polymerization of the material inpatterned imprinting layer 34 is known to one skilled in the art andtypically depends on the specific application which is desired.Furthermore, the plurality of features on patterned mold 28 are shown asrecesses 28 a extending along a direction parallel to projections 28 bthat provide a cross-section of patterned mold 28 with a shape of abattlement. However, recesses 28 a and projections 28 b may correspondto virtually any feature required to create an integrated circuit andmay be as small as a few tenths of nanometers.

It may be desired to manufacture components of system 10 from materialsthat are thermally stable, e.g., have a thermal expansion coefficient ofless than about 10 ppm/degree Centigrade at about room temperature (e.g.25 degrees Centigrade). In some embodiments, the material ofconstruction may have a thermal expansion coefficient of less than about10 ppm/degree Centigrade, or less than 1 ppm/degree Centigrade. To thatend, bridge supports 12, bridge 14, and/or stage support 16 may befabricated from one or more of the following materials: silicon carbide,iron alloys available under the trade-name INVAR®, or trade-name SUPERINVAR™, ceramics, including but not limited to ZERODUR® ceramic.Additionally, table 24 may be constructed to isolate the remainingcomponents of system 10 from vibrations in the surrounding environment.An exemplary table 24 is available from Newport Corporation of Irvine,Calif.

Referring to FIGS. 1, 2 and 5, the pattern produced by the presentpatterning technique may be transferred into wafer 31 provided featureshave aspect ratios as great as 30:1. To that end, one embodiment ofpatterned mold 28 has recesses 28 a defining an aspect ratio in a rangeof 1:1 to 10:1. Specifically, projections 28 b have a width W₁ in arange of about 10 nm to about 5000 μm, and recesses 28 a have a width W₂in a range of 10 nm to about 5000 μm. As a result, patterned mold 28and/or substrate 26, may be formed from various conventional materials,such as, but not limited to, fused-silica, quartz, silicon, organicpolymers, siloxane polymers, borosilicate glass, fluorocarbon polymers,metal, and combinations of the above.

Referring to FIGS. 1, 2 and 3, the characteristics of material 36 a areimportant to efficiently pattern wafer 31 in light of the uniquedeposition process employed. As mentioned above, material 36 a isdeposited on wafer 31 as a plurality of discrete and spaced-apart beads36. The combined volume of beads 36 is such that the material 36 a isdistributed appropriately over area of surface 32 where patternedimprinting layer 34 is to be formed. As a result, patterned imprintinglayer 34 is spread and patterned concurrently, with the pattern beingsubsequently set by exposure to radiation, such as ultravioletradiation. As a result of the deposition process it is desired thatmaterial 36 a have certain characteristics to facilitate rapid and evenspreading of material 36 a in beads 36 over surface 32 so that allthicknesses t₁ are substantially uniform and all residual thicknesses t₂are substantially uniform.

Referring to FIGS. 2 and 9, employing the compositions described abovein material 36 a, shown in FIG. 3, to facilitate imprint lithography isachieved by including, on substrate 131, a primer layer 46. Primer layer46 functions, inter alia, to provide a standard interface with patternedimprinting layer 34, thereby reducing the need to customize each processto the material from which substrate 131 is formed. In addition, primerlayer 46 may be formed from an organic material with the same etchcharacteristics as patterned imprinting layer 34. Primer layer 46 isfabricated in such a manner so as to possess a continuous, smooth,relatively defect-free surface that may exhibit excellent adhesion topatterned imprinting layer 34.

Additionally, to ensure that patterned imprinting layer 34 does notadhere to patterned mold 28, surface 28 c, shown in FIG. 5, may betreated with a low surface energy coating 48. As a result, patternedimprinting layer 34 is located between primer layer 46 and coating 48upon contact of mold 28 with substrate 131. Coating 48 may be appliedusing any known process. For example, processing techniques may includechemical vapor deposition method, physical vapor deposition, atomiclayer deposition or various other techniques, brazing and the like. In asimilar fashion a low surface energy coating 148 may be applied toplanarizing mold 128, shown in FIG. 10. Alternatively, releaseproperties of either patterned imprinting layer 34 or conformal layer140, shown in FIG. 11, may be improved by including, in the materialfrom which the same is fabricated, a compound having low surface energy,referred to as a surfactant. The compound is caused to migrate to asurface of the layer formed therewith to interface with mold 28 usingknown techniques. Typically, the surfactant has a surface energyassociated therewith that is lower than a surface energy of thepolymerizable material in the layer. An exemplary material and processby which to form the aforementioned surfactant is discussed by Bender etal. in Multiple Imprinting In UV-Based Nanoimprint Lithography: RelatedMaterial Issues, Microelectronic Engineering pp. 61-62 (2002). The lowsurface energy of the surfactant provides the desired release propertiesto reduce adherence of either imprinting layer 34 or conformal layer 40to mold 28. It should be understood that the surfactant may be used inconjunction with, or in lieu of, low surface energy coatings 48 and 148.

The embodiments of the present invention described above are exemplary.Many changes and modifications may be made to the disclosure recitedabove, while remaining within the scope of the invention. The scope ofthe invention should, therefore, be determined not with reference to theabove description, but instead should be determined with reference tothe appended claims along with their full scope of equivalents.

1. A method of patterning a substrate, the method comprising: forming apatterned layer on a substrate, the patterned layer having protrusionsand recessions; dispensing polymerizable material on the patternedlayer, defining a conformal layer, the patterned layer and the conformallayer collectively defining a multi-layered structure, the multi-layeredstructure having a crown surface facing away from the substrate; and,selectively removing portions of the multi-layered structure to exposeregions of the substrate in superimposition with the protrusions whileforming a hard mask in areas of the crown surface in superimpositionwith the recessions.
 2. The method as recited in claim 1, whereinforming the patterned layer further includes providing the protrusionswith an apex and a height.
 3. The method as recited in claim 2, whereindefining the conformal layer further includes providing the conformallayer with a normalization surface spaced-apart from one apex a minimumdistance and spaced-apart from an additional apex a maximum distancewith the height being greater than a difference between the maximumdistance and the minimum distance.
 4. The method as recited in claim 1,wherein the conformal layer is defined using silicon-containingpolymerizable material.
 5. The method as recited in claim 1, wherein theconformal layer is defined by dispensing, on the substrate, apolymerizable fluid composition and contacting the polymerizable fluidcomposition with a mold having a substantially planar surface andsubjecting the polymerizable fluid composition to conditions topolymerize the polymerizable fluid composition.
 6. The method as recitedin claim 1, further including depositing a primer layer between thepatterned layer and the substrate.
 7. The method as recited in claim 1,wherein the conformal layer is defined with a section having anormalization surface disposed opposite to the patterned layer andspaced-apart from the protrusions.
 8. The method as recited in claim 7,further including removing portions of the section to expose theprotrusions, defining the crown surface and exposing the crown surfaceto an etch chemistry.
 9. A method of patterning a substrate, the methodcomprising: creating a multi-layered structure with e-beam lithographyby forming, on the substrate, a patterned layer having protrusions andrecessions, by dispensing, on the substrate, droplets of polymerizablefluid composition and contacting the polymerizable fluid compositionwith a surface of a mold, and subjecting the polymerizable fluidcomposition to conditions to polymerize the polymerizable fluidcomposition, forming the patterned layer, and forming, upon thepatterned layer, a conformal layer, with the multi-layered structurehaving a crown surface facing away from the substrate; and selectivelyremoving portions of the multi-layered structure to expose regions ofthe substrate in superimposition with the protrusions while forming ahard mask in areas of the crown surface in superimposition with therecessions.
 10. The method as recited in claim 9, wherein forming thepatterned layer further includes providing the protrusions with an apexand a height.
 11. The method as recited in claim 10, wherein forming theconformal layer further includes providing the conformal layer with anormalization surface spaced-apart from one apex a minimum distance andspaced-apart from an additional apex a maximum distance with the heightbeing greater than a difference between the maximum distance and theminimum distance.
 12. The method as recited in claim 9, wherein creatingfurther includes forming the conformal layer from a silicon-containingpolymerizable material.
 13. The method as recited in claim 9, furtherincluding depositing a primer layer between the patterned layer and thesubstrate.
 14. The method as recited in claim 9, wherein creatingfurther includes forming the conformal layer with a section having anormalization surface disposed opposite to the patterned layer andspaced-apart from the protrusions.
 15. The method as recited in claim14, further including removing portions of the section to expose theprotrusions, defining the crown surface and exposing the crown surfaceto an etch chemistry.
 16. The method as recited in claim 15, whereinremoving portions further includes subjecting the normalization surfaceto a blanket etch and exposing the crown surface further includessubjecting the crown surface to an anisotropic plasma etch.